The present invention relates to digital signal processing, and more particularly, to multiplication of digital video signals.
Multiplication is a common signal processing function for video signals. Increasingly, video signals are in digital form, typically with 8-bit (256 grey scale levels) resolution. If it is desired to multiply an 8-bit digital signal by an 8-bit control signal (which can be another video signal, a shading signal, etc.), the resulting product signal has 16 bits (65,536 grey levels). Since a 16-bit signal cannot be transmitted through an 8-bit system, some of the output bits of the multiplier must be discarded. In particular, the 8 least significant bits would be discarded, leaving the 8 most significant bits to represent the product signal. Since the multiplier is capable of a 16-bit product signal, this discarding makes poor utilization of the relatively expensive multiplier.
One way of making a digital multiplier is to construct a look up table using a ROM (read only memory). The 8-bits each input and control signals can be considered as a single 16-bit address word, which word can "look up" 65,536 locations of data. But, again, if the product signal is restricted to an 8-bit word, then there are more locations of data than there are unique data words.
Another way of multiplying a signal is to use a microprocessor controlled RAM (random access memory) such as shown in U.S. patent application Ser. No. 286,264, filed July 23, 1981, in the name of R. A. Dischert and assigned to the assignee of the present application. In this system, the RAM is loaded with transfer coefficients (such as a multiplier) by the microprocessor (or, in an alternate embodiment, a hardwired circuit) during the vertical blanking interval or during several horizontal blanking intervals. Such a system may not be fast enough to do multiplication of a video signal where the multiplier signal is changing during a horizontal line, such as when the multiplier signal is a shading signal or another video signal.
It is therefore desirable to multiply large bandwidth signals in an inexpensive manner that makes full use of the circuits employed to do so.